A decalcomania (decal) usually is comprised of a multi-layer structure including a backing, a design or pigment layer, and a protective layer applied over the design layer. The colors in the design layer are formed from inorganic pigments or oxides. A layer facilitating release of the backing from the design may be interposed between the backing layer and the design layer. The protective layer applied over the design layer can comprise a low melting point glass which acts as a protective barrier over the surface of the design layer, which protective barrier, being glass, is resistant to both alkalis and acids, as well as to mechanical abrasion. Moreover, since this layer is transparent, the design and colors of the finished decal will appear as clearly as if no glass barrier were present.
A number of different types of decals are used at present in the pottery industry to apply patterns to ceramic ware. One of these is the so-called "underglaze" decal. This type of decal is applied to the ware after the ware has been formed but before it is glazed. Thereafter, a glaze is applied over the ware and decal. This glaze consists of a vitreous coating. The coating is formed directly from raw materials so that a very high temperature must be used in firing the ware to form the glass. The result of this process is a protective coating over the pigment such that the pigment will not be subject to chemical and mechanical attack such as produced by modern chemical detergents and mechanical washing devices. However, the use of such a high temperature as is necessary to form the glaze destroys the color value of many of the pigments that would be preferred to produce the desired colors. In addition, the glaze may be somewhat opaque so that the designs of the decal are obscured. It will be appreciated from the foregoing that this underglaze ceramic decal is limited in application.
As a result of these limitations, the so-called "overglaze" decals were developed. That is, decals which are applied to the ware after the high temperature glaze has been put on. These overglaze decals can generally be divided into two classes -- silk screen decals and lithographic decals.
In the silk screen process, a silk screen template or stencil is placed over the surface on which pigment is to be deposited and the pigment is applied through the screen. If the decal were of the "water mount" or "slide off" type, the surface on which the pigment is applied would be the layer of water soluble gum which has been placed over a paper backing. In this process, a relatively thick layer of pigment is deposited over the entire surface covered by the stencil. In order to increase the permanence of the design in the silk screen decal, a quantity of powdered low melting point glass may be mixed with the pigment so that, when the pattern is set by the application of heat, this powdered glass will fuse and become a part of the pattern itself. The silk screen decals, however, like the underglaze decals, are subject to a number of limitations. For example, the fine and clear cut designs and tonal variations available in the lithographic process cannot be obtained by the silk screen process. Also, the thicker pattern which results from this process is not always desirable. Because of these limitations lithographic decals are widely used in the industry.
Lithographic decals are formed by printing the desired pattern on the decal by a lithographic process. In the case of the water-mount decals, the pattern is printed on the top of the water soluble gum layer. As is the case with other types of decals it is essential that the pigment of lithographic decals be protected from the chemical and mechanical attack previously referred to. If it is not, the design will lose its lustre and brightness and the pigment may even rub off entirely with handling. The obvious solution to this problem would seem to be a mixture of powdered low melting glass with the pigments as in the silk screen process. There are a number of reasons why this is not normally done. As stated previously, in the case of the mixture, the number of pigments which could be used is limited since some are attacked by molten glass. In the lithographic process the thickness of the layer, and therefore the amount of pigment which can be applied is limited by the nature of the process itself. In addition, only about half the surface area of the decal surface is covered by the pigment dots in this process and so a higher concentration of pigment must be used. Therefore, if sufficient powdered glass to protect the design were mixed with the pigment in this process, the pigment would be so diluted that the color value would be seriously impaired or completely destroyed.
With the advent of modern chemical detergents and mechanical washing devices it is essential that the pigments of ceramic decals be protected from chemical and mechanical attack. That is, if the pigment were not protected it would fade, be abraded, and in many cases rub off. Thus, where possible, the use of a protective layer of glass is highly desirable and has been employed for the above purposes for years. Unfortunately, however, even the use of a protective barrier of glass has not been entirely satisfactory and, in fact, has significant drawbacks in certain applications. One of the primary applications of ceramic ware containing decorative decals resides in their use as household utensils such as in oven ware, casserole pans or dishes, plates, salad bowls, cups, mugs, other dishes and the like. During such use, the ware is subjected to extreme conditions such as widely varying temperatures, highly acidic or basic substances as well as even corrosive substances. After determinate periods of use under such conditions, subtle and in many instances even substantial changes can be observed in both the physical appearance and chemical make-up of the decorative decal. Although these changes may detract from the aesthetic apparance of the ware, this drawback is only a minor consideration as compared to the serious consequences which are possible as a direct result of such changes in the decal as will hereinafter be explained.
As indicated above, the colors in the design layer are formed from inorganic pigments including metallic oxides or sulfides. Colors obtainable from such pigments include yellow where lead oxide, lead chromate, cadmium oxide and/or cadmium sulfide are employed alone or in conjunction (e.g. as complexes) with other pigments; red where cadmium sulfoselenide and/or lead chromate are employed alone or in conjunction with (e.g. as complexes) other pigments, and white where lead molybdate is employed. Such lead and/or cadmium based pigments are toxic and thus human intake must be avoided.
The incidence of poisoning from intake if toxic metallic substances, such as lead found in lead-based paints, has been increasing in recent years at an alarming rate causing great concern especially among public officials and health authorities of large urban ceters where large pockets of sub-standard housing exist and lead-based paints have been employed. The result is that all articles and materials containing toxic metallic substances have become suspect, and especially those which come in contact with food and drink. Ceramic wares containing decorative decalcomanias of course fall into the latter category.
The use of protective barriers of glass over the design layer of decalcomanias has achieved some success in inhibiting release of toxic substances present in the design layer. However, it has been found that after repeated washings of ceramic ware, even containing such a protective barrier, at high temperatures employing caustic detergents or subjecting such ware to highly acidic and/or alkaline substances (such as polishes or even foods over long periods of time), some lead and possibly cadmium release through the protective glass barrier has been observed.
Kane, U.S. Pat. No. 2,734,840, discloses a lithographic overglaze decal which is strongly resistant to chemical and mechanical attack and which is made by dusting a mixture of powdered glass and solid pigment powder or solid pigment globules on a backing sheet containing an adhesive such as a gum or varnish to form a design layer thereon and thereafter covering the pigment-glass design layer with a layer of powdered low melting point glass. The decalcomania disclosed and claimed in the Kane patent has achieved wide acceptance and approval and, in fact, is the standard in the industry. The layer of powdered low melting point glass has been found to be somewhat successful in inhibiting lead and/or cadmium release from the design layer.
It has been found that the shortcomings of conventional protective barriers of glass, employed in decalcomanias, in inhibiting release of toxic materials from the design layer of the decalcomania are attributed primarily to the chemical make-up of the glass itself. Most glasses used as a protective barrier are lead boro-silicate glass containing various additives to build-in various desirable properties in the glass. For example, such glass may include zirconium dioxide (ZrO.sub.2) in amounts as high as up to 15% by weight or more to impart resistance to alkali attack, and titanium dioxide in amounts up to 15% or more to impart resistance to acid attack. It is theorized that zirconium and/or titanium can be employed in such large quantities because they are basically compatible with the other glass-forming constituents and fit nicely into the lattice or matrix structure of the glass. However, these metallic oxides are not entirely satisfactory because they tend to adversely affect the colors of the design layer. Attempts have been made to incorporate other materials into the glass matrix to overcome the shortcomings of the zirconium and titanium additives, without must success.
The applicants have invented a decalcomania including a protective barrier of a low melting point glass or frit and a method therefor, which overcomes the disadvantages attendant with prior art frits and decalcomania containing the same.